Vehicle lamps are used as lighting devices for illuminating the outside of vehicles to improve drivers' visibility or as display devices for transmitting various signals to the outside. Examples of vehicle lamps serving as lighting devices include headlamps and fog lamps having forward illumination light sources. In some cases, a vehicle lamp housing is provided with a vent member for providing an air passage to eliminate a pressure difference between the inside and outside of a lamp chamber. The vent member prevents entry of water and foreign matters such as dust into the lamp chamber while eliminating the pressure difference. A member including a waterproof air-permeable membrane is usually used as such a vent member.
For example, Patent Literature 1 discloses a headlamp shown in FIG. 8. A headlamp 401 includes a housing 402 and a light-transmitting member 403 attached to the front of the housing 402. A forward illumination light source 404 is disposed in a lamp chamber 406 enclosed by the housing 402 and the light-transmitting member 403. The housing 402 has opening portions 402A and 402B, and a microporous, air-permeable member as a vent member 405B is disposed over the opening portion 402B. A member 405A disposed over the opening portion 402A is a water-vapor-permeable but air-impermeable member for reducing condensation.
Water condensation is likely to occur to form water droplets (dew) on the inner surface of the light-transmitting member 403 because it is exposed directly to the outside air and thus is easily cooled. When the light source 404 is turned on, with the water droplets being formed on the inner surface of the light-transmitting member 403, air in the lamp chamber 406 expands as the temperature of the light-transmitting member 403 increases. Thus, a pressure difference is created between the inside and outside of the lamp chamber 406, but this pressure difference is eliminated by the air-permeable vent member 405B. In addition, water vapor produced by evaporation of the water droplets is released not only through the vent member 405B but also through the member 405A serving as a condensation reducing member. The member 405A is not a “vent member” because it is air impermeable and thus does not help to eliminate a pressure difference, but it promotes elimination of condensation.